1. Field of the Invention
The present invention relates to a cylindrical secondary battery, and more particularly, to a cylindrical secondary battery having a one-way valve in a cap assembly to exhaust gas generated during charging the battery.
2. Description of the Related Art
Batteries are electrical products for supplying a DC voltage. There are various types of batteries used for various electrical or electronic products and devices and rechargeable batteries used for vehicles and as emergency power for mechanical apparatuses. Batteries which are not rechargeable are generally referred to as primary batteries while ones capable of discharge and recharge by being provided with electricity from other electric powers are referred to as secondary batteries.
FIG. 1 is a sectional view showing the structure of a typical cylindrical secondary battery, Referring to the drawing a conventional secondary battery 10 includes a case 14 of a cylindrical can shape and an anode plate 16 and a cathode plate 17 installed in the case 14. The anode plate 16 and the cathode plate 17 are formed in strips having predetermined widths and lengths. A separator 18 is interposed between the anode plate 16 and the cathode plate 17 to prevent the anode plate 16 and the cathode plate 17 from contacting each other. In such a state, the anode plate 16 and the cathode plate 17 are rolled forming a jelly-roll.
Thus, the anode plate 16 and the cathode plate 17, which are rolled with interposition of the separator 18, alternate. Upper and lower ring members 15a and 15b supporting an insulation member 18 are installed at the upper end and the lower end of a jelly-roll of the anode plate 16 and the cathode plate 17, respectively.
A gasket 13 is arranged above the upper ring member 15a and a cap cover 12 and a cap 11 supported by the gasket 13 are provided. A fragile portion 19 is formed at the central position of the cap cover 13a. The fragile portion 19 is integrally formed with the cap cover 12 and, when the pressure inside the case 14 increases, the fragile portion 19 is broken to exhaust gas.
FIG. 2 is an exploded perspective view showing a cap assembly of the battery shown in FIG. 1. Referring to the drawing, the fragile portion 19 is located at the central portion of the cap cover 12 as described above and a bending portion 12a, which is bent during assembly, is formed at the edge of the cap cover 11. The fragile portion 19 is formed by forming a notch or a thin portion at the central portion of the cap cover 12 to partially decrease breaking strength. Thus, when the pressure in the case 14 increases, the fragile portion 19 is broken so that gas can be exhausted.
The process of manufacturing a battery having the above structure includes steps of inserting the anode plate 16 and the cathode plate 17 in the case 14, injecting electrolyte, and arranging a cap assembly including the cap 11 above the case 14. In performing a sealing process with respect to the cap assembly, a cathode tap at the cathode plate 17 is welded at a predetermined position of the cap assembly so that a stable electrical connection to the cathode plate 17 is maintained. Contrarily, as the anode plate 16 directly contacts the inner surface of the case 14, an electrical connection is achieved.
A charge process is performed before completed batteries are shipped as products. That is, secondary batteries are charged to a degree before they are shipped as products. Typically, it is known that gas is generated most in a secondary battery when the initial charge is performed. As gases generated during the initial charge, there are hydrogen carbon monoxide, carbon dioxide and hydrocarbons. As other gases are generated in addition to the above, there is a possibility of explosion. However, there has not been a method for effectively exhausting the generated gas. That is, although gas can be exhausted to the outside by forming the fragile portion 19 at the cap cover 12 and breaking the fragile portion 19 when the gas pressure increases over a predetermined value of pressure, if the fragile portion 19 is broken, the cap assembly cannot be used again.